11.10 Buffer-Local Variables

Global and local variable bindings are found in most programming
languages in one form or another. Emacs, however, also supports
additional, unusual kinds of variable binding, such as
buffer-local bindings, which apply only in one buffer. Having
different values for a variable in different buffers is an important
customization method. (Variables can also have bindings that are
local to each terminal. See Multiple Terminals.)